UBC Theses and Dissertations
Transfer RNA genes in Drosophila melanogaster Newton, Craig Hunter
In Drosophila melanogaster the 600-800 reiterated copies of genes encoding cytoplasmic tRNA are dispersed across the genome at a multitude of chromosomal sites. In many cases these sites contain several copies of the same and different tRNA genes. Here we describe tRNA genes obtained from two of these sites; one, on the X chromosome at 12E, is the major site of in situ hybridization with tRNA₇[sup Ser] (anticodon IGA) or tRNA₄[sup Ser] (CGA). The second is one of three minor hybridization sites and is located on chromosome 2L at 23E . DNA sequencing studies here show that the plasmid derived from 12E (pDt27R) contains two identical tRNA₄[sup Ser] genes and four identical genes encoding a minor arginine isoacceptor (UCG). Previous studies with different plasmids had shown 12E also contains two genes identical to tRNA₇[sup Ser] and two additional one and two basepair variants thereof. Thus 12E contains at least ten genes encoding three, or perhaps five, different isoacceptors. The organization of these genes within 12E is not known. In pDt27R all six genes occur within 2.3 kbp and are in the same transcriptional orientation. The two serine genes are 317 bp apart and are separated by 669 bp from the four adjacent arginine genes. While little flanking sequence homology occurs around the serine genes, the flanking sequence of each arginine gene is almost identical. The extent of these homologies differ mainly in the amount of shared 5' flanking sequence; two genes are contained on repeats with 455 bp of common sequence while two others are contained on repeats that share only 30 bp of 51 flanking sequence. All four genes have the same 75 bp of 3' flanking sequence. The repeats are tandemly linked by junction sequences 5-27 bp in length. The flanking sequence around one gene is considerably more divergent (18% versus 1-3%) from the other three. This structure suggests that the gene quartet arose from a single primordial gene by a series of duplication events that included identical and different amounts of flanking sequence. In one case the duplication included identical amounts of flanking sequence but acted at times distant enough for 18% divergence to accumulate. The plasmid bearing DNA from 23E on chromosome 2L (pDt5) contains a single gene identical to the tRNA₇[sup Ser] genes previously described at 12E on the X chromosome. This gene occurs alone approximately in the middle of a 4.4 kbp fragment of Drosophila DNA. The only similarities in flanking sequence with the two identical genes at 12E are small regions that occur within 30 bp of the 5' mature coding sequence. Similarly, different sequences are conserved at analogous positions 5' to the two tRNA₄[sup Ser] genes also at 12E.
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